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High Efficiency and Large-Scale Subsurface Energy Storage with CO2

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High Efficiency and Large-Scale Subsurface Energy Storage with CO2 PROCEEDINGS, 43rd Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 12-14, 2018 SGP-TR-213 High Efficiency and Large-scale Subsurface Energy Storage with CO2 Mark R. Fleming1, Benjamin M. Adams2, Jimmy B. Randolph1,3, Jonathan D. Ogland-Hand4, Thomas H. Kuehn1, Thomas A. Buscheck3, Jeffrey M. Bielicki4, Martin O. Saar*1,2 1University of Minnesota, Minneapolis, MN, USA; 2ETH-Zurich, Zurich, Switzerland; 3TerraCOH Inc., Minneapolis, MN, USA; 4The Ohio State University, Columbus, OH, USA; 5Lawrence Livermore National Laboratory, Livermore, CA, USA. *Corresponding Author: [email protected] Keywords: geothermal energy, multi-level geothermal systems, sedimentary basin, carbon dioxide, electricity generation, energy storage, power plot, CPG ABSTRACT Storing large amounts of intermittently produced solar or wind power for later, when there is a lack of sunlight or wind, is one of society’s biggest challenges when attempting to decarbonize energy systems. Traditional energy storage technologies tend to suffer from relatively low efficiencies, severe environmental concerns, and limited scale both in capacity and time. Subsurface energy storage can solve the drawbacks of many other energy storage approaches, as it can be large scale in capacity and time, environmentally benign, and highly efficient. When CO2 is used as the (pressure) energy storage medium in reservoirs underneath caprocks at depths of at least ~1 km (to ensure the CO2 is in its supercritical state), the energy generated after the energy storage operation can be greater than the energy stored. This is possible if reservoir temperatures and CO2 storage durations combine to result in more geothermal energy input into the CO2 at depth than what the CO2 pumps at the surface (and other machinery) consume. Such subsurface energy storage is typically also large scale in capacity (due to typical reservoir sizes, potentially enabling storing excess power from a substantial portion of the power grid) and in time (even enabling seasonal energy storage). Here, we present subsurface electricity energy storage with supercritical carbon dioxide (CO2) called CO2-Plume Geothermal Energy Storage (CPGES) and discuss the system’s performance, as well as its advantages and disadvantages, compared to other energy storage options. Our investigated system consists of a deep and a shallow reservoir, where excess electricity from the grid is stored by producing CO2 from the shallow reservoir and injecting it into the deep reservoir, storing the energy in the form of pressure and heat. When energy is needed, the geothermally heated CO2 is produced from the deep reservoir and injected into the shallow reservoir, passing through a power generation system along the way. Thus, the shallow reservoir takes the place of a storage tank at the surface. The shallow reservoir well system is a huff-and-puff system to store the CO2 with as few heat and pressure losses as possible, whereas the deep reservoir has an injection and a production well, so the CO2 can extract heat as it passes through. We find that both the diurnal (daily) and seasonal (6 months) CPGES systems generate more electricity to the power grid than they store from it. The diurnal system has a ratio of generated electricity to stored electricity (called the Energy Storage Ratio) between 2.93 and 1.95. Similarly, the seasonal system has an energy storage ratio between 1.55 and 1.05, depending on operational strategy. The energy storage ratio decreases with duration due to the pump power needed to overcome the increasing reservoir pressures as CO2 is stored. 1. INTRODUCTION The development of modern electricity systems which reduce the amount of carbon dioxide (CO2) emitted into the atmosphere while producing steady, continuous power is one of society’s biggest challenges. To limit the global mean temperature rise to 2°C, the Intergovernmental Panel on Climate Change (IPCC) has estimated that an atmospheric limit of 250 ppm of CO2 results in a 50% chance of obtaining this temperature goal (IPCC 2014a). These regulations were agreed upon by a majority of nations in the Paris Agreement (United Nations Framework Convention on Climate Change 2015), allowing for an estimated 1000 GT of CO2 to be emitted after 2011 (IPCC 2014b). This requires the immediate reduction, and eventual elimination of CO2 emissions, to avoid exceeding this CO2 emission limit. No single technology will provide the necessary reduction and elimination of CO2 emissions; however, multiple technologies employed and integrated as a whole can provide the necessary reduction in CO2 emissions. To reduce CO2 emissions in the electricity sector, which accounts for 25% of the total CO2 emissions, existing power plants can be retrofitted with CO2 capture technologies and carbon-neutral power systems can replace existing generation (IPCC 2014a; Metz et al. 2005). To decarbonize existing power plants, CO2 emissions can be captured, transported, typically via a pipeline, to a storage site, and then injected into a subsurface reservoir, in a process referred to Carbon Capture and Storage (CCS). CCS reduces the emission of CO2 into the atmosphere from sources such as fossil fuel power systems, cement factories, biofuel refineries, or from other large CO2 point sources by permanently storing the CO2 underground in deep saline aquifers or partially depleted oil/gas fields, which can store large volumes of CO2. The vertical leakage of the captured CO2, which is naturally buoyant at the storage conditions, is contained by the overlying low permeability caprock. In addition to structural trapping, CO2 is stored in the reservoir due to capillary forces, dissolution into the underlying brine, and eventually the formation of carbonate minerals. Due to the depth of the storage formation, which is generally in excess of 800 meters to ensure supercritical CO2 and maximize storage volumes, the average reservoir temperatures are greater than the temperature of the injected CO2, and can be significantly greater, depending on the geothermal gradient, than the surface temperature, thus allowing the injected CO2 to extract heat from the reservoir. This heat extraction process has led to the 1 Fleming et al. proposal of geothermal energy systems which can be combined with CCS, such as CO2-Plume Geothermal (CPG), which directly uses the CO2 as the heat extraction fluid, thereby operating as a Carbon Capture Utilization and Storage (CCUS) system, discussed in detail below. In addition to CCS, renewable energy sources, such as wind and solar, can provide energy without fossil fuels, and their associated CO2 emissions; however, these sources are variable power systems, capable of producing power only when the given resources are available. In 2016, wind had an annual capacity factor of 34.5%, while solar photovoltaic had a 25.1% capacity (EIA 2017), due to the variability of their resources. The intermittent nature of these resources can provide challenges integrating these technologies into existing electrical grids by creating an excess or deficit in power generation, reducing the efficiency of the grid (Bird et al. 2013; Phuangpornpitak and Tia 2013). To provide baseload power, energy storage systems can be integrated with the intermittent renewable sources to store excess power when it is generated, producing the energy at a later period when there is a demand for power (Koohi- Kamali et al. 2013; Sørensen 2015). With the expanded capacity of wind and solar, additional energy storage capacity is required to ensure electrical grid reliability. However, existing large-scale bulk energy storage systems, such as Pumped Hydroelectric and Compressed Air, may not have the ability to provide the expanded capacity that is required. Pumped Hydroelectric systems, have limited development opportunities resulting from environmental concerns regarding the development of the large surface storage reservoirs. Additionally, compressed air does not represent a sustainable long-term energy storage solution, as compressed air relies on auxiliary surface heating, typically from natural gas, to produce power, emitting CO2 in the process. While these energy storage technologies have limitations, geothermal energy is widely available and can be accessed by CO2-Plume Geothermal systems, which can be used to supplement wind and solar. CO2-Plume Geothermal (CPG) systems operate by producing hot CO2, which is geothermally heated in a natural high permeability reservoir, to the surface for power, or heat, generation (Adams et al. 2014, 2015, Randolph and Saar 2011a, 2011b, 2011c). The produced CO2 is then reinjected into the reservoir, in a cold dense state, allowing the injected CO2 to extract heat from the reservoir. CPG is different than CO2-Enhanced Geothermal Systems (CO2-EGS), which have previously been studied (Atrens et al. 2009, 2010; Brown W. 2000; Pruess 2008), as the CPG system uses natural high permeability sedimentary basins with a large storage volume, whereas CO2-EGS requires artificially generated high permeability reservoirs which are generally small and offer limited CO2 storage capacity. Operating a geothermal power system using CO2 has several advantages, beyond the synergistic power production from a CCS site, including a low mineral solubility, high reservoir mobility (low kinematic viscosity), and a large density variation with temperature. A low mineral solubility is advantageous, as the produced fluid will contain minimal impurities and pipe scaling will be limited.
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